Successful hemodialysis and hemofiltration require access to the circulatory system, often by means of an arteriovenous fistula or graft. Extracorporeal blood flow rates above 200 ml/min are essential for efficient hemodialysis and hemofiltration. Many patients experience dialysis vascular access (e.g., fistula) problems which produce extracorporeal blood flow rates that are suboptimal. If vascular access cannot supply fresh blood at a rate demanded by the dialysis blood pump, a portion of blood returning from the dialysis machine, via a venous line, recirculates back to the machine via the arterial line, instead of returning to the patient. The quantity of this recirculation will directly influence the efficiency of the dialysis process. Ideally, zero recirculation is desired and when this recirculation is in excess of 10% of returning blood, the efficiency of dialysis is adversely affected.
One essential element for improving dialysis patient care is the effective and speedy detection and measurement of recirculation. Recirculation during dialysis can be caused by a variety of factors, such as: (1) improper placement of needles in the vascular access, (2) narrowing (stenosis) of fistulae or graft structure, (3) improper position of catheter ends, or (4) mechanical obstruction of venous line or venous limbs of the vascular access. Despite this common problem, there has been no satisfactory, inexpensive, simple and objective method of assessing recirculation.
Over the past decade, there has been a deliberate attempt to increase the efficiency of dialysis by increasing blood flow rates in order to reduce dialysis treatment times. For such modes of high efficiency dialysis, a blood flow rate in excess of 300 ml/min is required. However, this high blood flow rate may increase the amount of recirculation, directly impacting the efficiency of dialysis.
One method for detecting recirculation is the saline bolus injection method described by Greenwood et al., Clinical Nephrology 23:189, 1985. This method involves a chilled saline bolus injected into the venous blood line that can be detected by a drop in temperature in the arterial line in the presence of recirculation. According to a later article by the same group of authors (Tattersall et al., Nephrol. Dial. Transplant. 8:60, 1993), the method "requires special equipment such as the fistula assessment monitor (FAM, Gambro)." This method requires that the pumps be adjusted downward, initially to about 100 ml/min, in order to make the recirculation measurement determination. This procedure, in itself, is time consuming and greatly increases dialysis time because if recirculation is determined in the first assessment, then the procedure will have to be repeated several times in a trial and error procedure when adjusting the needles or the pump speed. Moreover, there is an increased risk of infection due to injections in access lines. Variability in the rate of manual injection of the saline bolus also introduces error to the recirculation measurement.
Recently, Tattersall et al., infra describe "a simpler method" for detecting and measuring recirculation. This method involves simultaneous sampling of blood (three sites) from the arterial and venous lines and from a peripheral vein. Each sample is submitted to a clinical laboratory for a urea determination. It is assumed that if there is recirculation within the blood access, the urea concentration in the arterial line is lower than in the peripheral vein because of mixing with dialyzed blood which has recirculated from the venous access device. The fraction of blood entering the arterial access device which has recirculated directly from the venous access device is termed the recirculation fraction (RF) and is calculated from the three measured urea fractions. This has become the most commonly used procedure to calculate recirculation. Also, the urea measurements require a clinical lab (and considerable expense) and the results may not be obtained until the next day, long after the dialysis procedure has been completed. However, there is increased variability due to clinical laboratory errors in measuring BUN (blood urea nitrogen).
Therefore, there is a need in the art for a rapid recirculation detection procedure and device that provides real time information and that enables convenient multiple measurements of recirculation following needle or equipment adjustment. There is a further need in the art for a recirculation measurement method that does not use expensive equipment, introduce an invasive procedure, laboratory analysis, or require a break in the dialysis method to take a recirculation measurement. The present invention was made to address each of the foregoing unmet needs.